The physiological costs of reproduction in small … physiological costs of reproduction in small mammals John R. Speakman* Aberdeen Centre for Energy Regulation and Obesity (ACERO),

Phil. Trans. R. Soc. B (2008) 363, 375–398

doi:10.1098/rstb.2007.2145

The physiological costs of reproductionin small mammals

Published online 8 August 2007

John R. Speakman*

One concycle’.

*j.speak

Aberdeen Centre for Energy Regulation and Obesity (ACERO), School of Biological Sciences,University of Aberdeen, Aberdeen AB24 2TZ, UK

Life-history trade-offs between components of fitness arise because reproduction entails both gainsand costs. Costs of reproduction can be divided into ecological and physiological costs. The latterhave been rarely studied yet are probably a dominant component of the effect. A deeperunderstanding of life-history evolution will only come about once these physiological costs arebetter understood. Physiological costs may be direct or indirect. Direct costs include the energy andnutrient demands of the reproductive event, and the morphological changes that are necessary tofacilitate achieving these demands. Indirect costs may be optional ‘compensatory costs’ whereby theanimal chooses to reduce investment in some other aspect of its physiology to maximize the input ofresource to reproduction. Such costs may be distinguished from consequential costs that are aninescapable consequence of the reproductive event. In small mammals, the direct costs ofreproduction involve increased energy, protein and calcium demands during pregnancy, but mostparticularly during lactation. Organ remodelling is necessary to achieve the high demands of lactationand involves growth of the alimentary tract and associated organs such as the liver and pancreas.Compensatory indirect costs include reductions in thermogenesis, immune function and physicalactivity. Obligatory consequential costs include hyperthermia, bone loss, disruption of sleep patternsand oxidative stress. This is unlikely to be a complete list. Our knowledge of these physiological costsis currently at best described as rudimentary. For some, we do not even know whether they arecompensatory or obligatory. For almost all of them, we have no idea of exact mechanisms or howthese costs translate into fitness trade-offs.

Keywords: energy; protein; calcium; pregnancy; lactation; life-history

1. INTRODUCTIONThe theory of evolution by natural selection assumes

that an individual’s attributes result from their

historical attempts to maximize fitness—their genetic

contribution to the future gene pool. Fitness, however,

is not a singular concept, but rather consists of a

number of components. The life history of an organism

is a reflection of how the animal blends together these

components into an integrated life-history ‘strategy’. A

fundamental axiom of our understanding and interpre-

tation of the evolution of life-history strategies is that

trade-offs exist between the various life-history com-

ponents (Fisher 1930). For example, an increase in

current reproductive output by maximizing fecundity

might occur only at the expense of future survivorship

or future reproductive output (Williams 1966; Stearns

1992; Charnov 1993; Charlesworth 1994). We assume

that animals cannot combine together the fitness

components in a strategy that simultaneously maxi-

mizes them all, and must therefore trade-off the

components against each other (but see Johnston

et al. 2006).

The idea that there must be trade-offs in fitness

components stems from the idea that reproductive

tribution of 14 to a Theme Issue ‘Adaptation to the annual

[email protected]

375

events generate fitness benefits in terms of viableoffspring, but that reproduction also entails costs aswell. These costs drive the nature of the trade-offbetween the fitness components (Reznick et al. 1990;Roff 1992). There are several different types of trade-offamong components of current reproductive effort, forexample, trading off the size against number ofoffspring (Smith & Fretwell 1974) and also trade-offsbetween current reproduction and components offuture fitness: either survival or fecundity. Stearns(1992) identified 10 different fitness components orlife-history ‘traits’ and detailed a matrix of potentialtrade-offs between them. For each trade-off, there mustbe a mechanism that links the fitness componentstogether. This mechanism is the ‘cost’ that is attachedto the reproductive event. Previous investigators haveidentified that these costs can be further partitionedinto physiological costs, and costs that are mediated viaecology (e.g. Zera & Harshman 2001). An example ofan ecological cost is the increased risk of predation thatis associated with foraging while acquiring energy andnutrients, like protein and calcium, for the reproduc-tive event. Historically, the study of life histories hasbeen rooted in the determination of the magnitude oftrade-offs with little concern for their mechanisticbasis. However, it is becoming increasingly recognizedthat distinguishing the importance of ecological andphysiological mechanisms is a key goal if we are tounderstand the evolution of life-history patterns

This journal is q 2007 The Royal Society

376 J. R. Speakman Costs of reproduction in small mammals

(Zera & Harshman 2001; McNamara & Houston2008). Nevertheless, it should be borne in mind thatecological and physiological components stronglyinteract and cannot be completely isolated from eachother. Despite the fact that it is widely recognized thatphysiological costs underpin all life-history trade-offs(e.g. Stearns 1992; Ricklefs & Wikelski 2002), thephysiological contribution to most, if not all, trade-offsremains very poorly studied. In this paper, I propose toreview our understanding of some of the physiologicalcosts that are attached to the process of reproduction.I have focused the review on small mammals wherethere is at least some depth of study, but I also confessthat this betrays a bias towards a group of animals thatI have studied personally.

Physiological costs of reproduction can be dividedinto two different types. The first is direct costs thatstem from satisfying the demands of the reproductiveevent itself. These demands at their simplest level arethe energy and nutrients that the parental animal needsto acquire to successfully reproduce. In the previousparagraph, I identified the predation risk that attendsthe extra foraging required to collect the energy ornutrients required for reproduction as an ecologicalcost. The direct physiological cost of reproduction isthat energy and nutrient demand. It is probable thatthese direct costs have deeper layers of complexity tothem. Animals not only require energy and the majormacronutrients (like protein) but also essential aminoand fatty acids, as well as intakes of vitamins andmicronutrients. Our knowledge of many of thesephysiological requirements is almost non-existent. Ihave also included in this ‘direct physiological cost’ thephysiological and anatomical modifications that arenecessary for the animal to achieve these demands. Forexample, if an animal needs to double or treble its rateof food intake to satisfy the demand for energy duringreproduction, it cannot simply do that without somemodifications to its alimentary tract and the machinerythat processes ingested food. These necessary modifi-cations are a second form of direct cost.

The second type of cost of reproduction is indirect.Like direct costs, we can also subdivide theindirect costs into two sub-types. First, to meet thedirect physiological costs of reproduction, an animalmay cut corners in other components of its physiology.Many animals probably inhabit environments where itis simply not possible for their intake to be increasedadequately to cover their reproductive demands. Inthese circumstances, the animals may make compensa-tory adjustments in aspects of their physiology to saveenergy that can then be devoted to reproduction. Evenwhere food is sufficiently available, there may beecological costs involved in obtaining it (e.g. predationrisks), which are greater than the physiological costs ofcutting down the expenditure on other components ofphysiological functioning. These compensatory phys-iological costs are an indirect cost of the reproductiveevent. A distinguishing feature of these costs is that theyhave a degree of optionality about them. The animal ischoosing to modify its physiology to invest more inreproduction, and it can choose to make this allocationof resource or not. The costs derive from the fact thatresource is limited and animals must therefore choose

Phil. Trans. R. Soc. B (2008)

how to allocate this limited resource between compet-ing functions. We can distinguish these optional‘compensatory costs’ from other indirect costs thatare an inevitable physiological consequence of thereproductive event. For example, reproduction mightinvolve the production of a toxic by-product thatelevates the risk of the animal developing a fatal diseaselike cancer. This would be a cost of reproduction thatwould not be optional. The animal could not choose toreproduce without this negative consequence.

2. DIRECT COSTS(a) Increased demand for energy and nutrients

(i) EnergyProbably the most detailed data available for the energydemands of reproduction are derived from domesti-cated rodents. This is primarily because these animalsare easily kept, and even quite invasive measurementscan be made on them without the risk that they willdesert their offspring. The time course of food intake, at218C, throughout reproduction in a strain of laboratorymouse (MF1) that we have been studying is shown infigure 1a (Johnson et al. 2001a). Food intake increasedduring pregnancy to approximately 8 g, compared with5.5 g in the non-breeding females prior to reproduc-tion. Although the pattern of foetal growth is essentiallyexponential between conception and birth, the patternof food intake does not mirror this, but rather rises to apeak approximately 2–3 days earlier and then declinesslightly before the day of parturition. The reason forthis pattern is uncertain, but one hypothesis is that theexpanding foetal mass competes for space with thealimentary tract in the abdomen, limiting the foodintake. This may suggest that resources to support thegestation become limited in late pregnancy. Undersuch limitation, competing demands may have detri-mental effects on the success of the pregnancy. Onesuch competing demand may be the level of basalenergy requirements (basal metabolic rate, BMR). Wehave recently shown in another strain of mouse (theC57BL/6 black mouse) that mice with higher BMRhave a greater likelihood of mass anomalies occurringduring pregnancy. These mass anomalies probablyreflect foetal resorption events. Because BMR iscorrelated with body mass, this relationship mightonly be an artefact of both resorption rate and BMRbeing affected by mass, but the effect of BMR is alsoevident if the effect of mass is removed statistically( Johnston et al. 2007). This effect supports thehypothesis that resource intake in late pregnancy maybe limited, perhaps by space competition in theabdomen between the alimentary tract and thedeveloping foetuses.

The most dramatic increase in food intake occurredduring lactation. During the initial 10 days, thisincrease was linear, but then it reached a plateau atapproximately 23 g of food per day. Food intake at theplateau (between days 10 and 18) was related to littersize. Small litters reached plateau intakes of less than23 g (figure 1b), but as the litter size increased foodintake also increased to a plateau at approximately 23 gof food per day. The mice seemed to reach a limit in

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Figure 1. (a) Food intake each day throughout pre-breeding,pregnancy and lactation phases for MF1 mice. Data averagedacross 71 litters. (b) Daily food intake averaged over the lastdays of lactation (10–18) plotted against litter size in MF1mice. (c) Pup mass in relation to litter size in MF1 mice.(d ) Histogram showing the frequency at birth of differentlitter sizes across 71 litters of MF1 mice.

Costs of reproduction in small mammals J. R. Speakman 377

their food intake at this level. Although litter sizes

increased, food intake did not increase in parallel.

We have shown that this limit in food intake is not

mediated via the aspects of the cage the animals live in

(Speakman & Krol 2005), and therefore appears to be a

physiological feature intrinsic to the animals them-

selves. This apparent physiological limit in the capacity

of the mouse to ingest food at peak lactation may

underpin an important life-history trait (maximum

litter size) and an important life-history trade-off. Since


the maximum asymptotic food intake at peak lactationis fixed, the energy that can be devoted to milk is alsofixed. As the litter size increases, this milk must bedivided between more and more offspring and,consequently, the pups wean at progressively smallerbody masses (figure 1c; Johnson et al. 2001a). Aphysiological limit in the capacity to ingest foodtherefore appears to underpin the life-history trade-offbetween the number and size of offspring. Presumably,there is a minimum size of offspring at weaning, whichwould stand any chance of survival. Where thedeclining relationship between pup size and litter sizeintersects, this theoretical minimum viable pup sizemay define the maximum litter size. A distribution ofnatural litter sizes in this mouse strain is shown infigure 1d. It is noticeable that the asymptotic intakes forthe mice that raised 14 and 15 pups were actuallyhigher than the average 23 g limit (figure 1d ). Perhapsthese females were only able to raise such large littersbecause they were individuals that were capable ofabove-average intakes, and therefore still able to weantheir pups at a pup mass of 7.5 g. Natural litter sizes of16 and above in this strain may not be feasible owing tothe interaction between the limit on maximal intakerate and the minimum viable pup size (this minimumviable pup size at weaning may also be physiologicallymediated).

Understanding the physiological basis of thismaximal intake limit is therefore of critical importance.Two hypotheses concerning the nature of this limitwere proposed in the early 1990s (Peterson et al. 1990;Weiner 1992; Hammond & Diamond 1997). The firsthypothesis was that the limit was imposed by thecapacity of the alimentary tract to absorb food (energy)and process it into a form for mobilization. This wascalled the ‘central limit hypothesis’. The secondhypothesis was that the limit is imposed at theperipheral site where the centrally supplied energy isused: the mammary glands. This was called theperipheral limit hypothesis.

Hammond & Diamond (1992) manipulated littersof Swiss Webster mice and found that females did notelevate their food intake when given up to 23 pups.Similarly, we have also shown that MF1 mice given upto 19 pups also could not breach the 23 g limit that theyreached during unmanipulated lactations with litters ofgreater than 10 offspring (Johnson et al. 2001a). Facedwith this problem of a litter size that is ‘too large’,females will often cull their offspring rather than eatmore food (Johnson et al. 2001a; Gandelman & Simon1978). In a separate experiment, Hammond &Diamond (1994) prevented pups from weaning attheir normal weaning age and hence their increasinggrowth demands needed to be supplied by the motheruntil the pups were 24 days old. In these conditions, themothers also could not upregulate their food intake.The absence of an increase in the intake when pupnumbers are increased or lactation extended confirmsthere is a physiologically imposed limit, but does notseparate between the peripheral and central limitshypotheses.

Giving female mice additional energy-demandingtasks during late lactation can better separate thealternative hypotheses. The basis of these experiments


is that, if the system is centrally limited, the total energyavailable will be fixed. Using some of this energy toperform another task will reduce the amount availableto support milk production and the consequence willbe a diminution of reproductive output. Alternatively,if the system is peripherally limited, the animals willincrease their intakes to meet the combined demands,and reproductive output will remain unaffected. Threemanipulations have been performed to test these ideas:lactating mice have been forced to exercise to obtaintheir food; they have been made simultaneouslypregnant, combining the demands detailed infigure 1a, and they have been exposed to cold.

Perrigo (1987) compared the reproductive strategiesof house mice Mus domesticus and deer mice Peromyscusmaniculatus by forcing females to run a preset number ofrevolutions (between 75 and 275) on a wheel to obtaineach pellet of food. Despite the combined demands oflactation and locomotor activity, neither house nor deermice exceeded the upper limit of food intake comparedwith unmanipulated mothers, given free access to food.As a result of the decreased amount of energy availablefor reproduction, the wheel-running house miceroutinely killed some of the offspring throughout thefirst 12 days of lactation, whereas deer mice extendedlactation well beyond normal weaning age.

Johnson et al. (2001b) followed the intakes of micethat had been mated immediately post-partum andfound that the mice concurrently lactating and pregnantdid not respond to the increased energy burden byelevating their food intake. Instead, they delayedimplantation at the start of the second pregnancy andthe length of this delay was directly related to thenumbers of pups. The animals therefore ‘avoided’overlapping their energy demands, perhaps becausethey could not elevate their total intake at peak lactationto accommodate both. Similar observations were madein Rockland-Swiss mice (Biggerstaff & Mann 1992),and in rats (Rattus norvegicus: Koiter et al. 1999), whenfood intake in late lactation was actually reduced inthose rats that carried a simultaneous pregnancy,relative to rats just lactating. Together, these activityand pregnancy studies indicate that the limits arecentrally, rather than peripherally, mediated; although,in case of concurrent pregnancy, the evidence is lessstrong as the animals avoided the problem.

However, when Hammond et al. (1994) exposedlactating Swiss Webster mice to 88C (approx. 228Cbelow the lower critical temperature), they found thatfood intake increased dramatically beyond the sup-posed centrally imposed limit. Similar observationshave since been made in deer mice (Hammond &Kristan 2000), MF1 mice (Johnson & Speakman2001) and cotton rats (Sigmodon hispidus; Rogowitz1998). The capacity of the mice to elevate their foodintake in the cold was completely at odds with thecentral limitation hypothesis. To test whether the limitwas imposed peripherally, Hammond et al. (1996)experimentally manipulated female mice by surgicallyremoving some of their mammary glands during latelactation. The rationale behind this experiment wasthat if the capacity of the mammary glands was limited,then, if the mammary tissue was halved in size, theremaining tissue would be unable to compensate by


elevating its milk production. However, if the capacityof the tissue was flexible and limited only by thecentrally controlled supply of energy, then it wouldrespond to the absence of half the tissue by expandingits capacity. They found that productivity in the halvedglands did not increase, suggesting that the mammarygland was indeed the point at which the system wasperipherally limited.

The above experiments suggested that there is aphysiological limit in the capacity of the mammarytissue to secrete milk which underlies the asymptoticfood intake in late lactation, and thus the trade-offbetween litter size and pup size and potentially definesthe maximal litter size. When animals were manipu-lated at room temperature by giving them more pups toraise, food intake did not increase because milkproduction was limited by the capacity of themammary glands. When exposed to cold conditions,food intake did increase (demonstrating a lack ofcentral limitation) owing to the combined demands formilk production (at maximal capacity) and increasedthermogenesis.

Several other studies have been performed, whichalso indicate that the limits to intake at peak lactationare not centrally mediated. These studies involvemanipulation of the energy density of the food. If theenergy density of food is decreased, but animals havea central processing capacity limit, they should beunable to upregulate their intake to compensate.Speakman et al. (2001) fed MF1 mice a diet thatprovided 25% less digestible energy than their normalfood and then mated them. Food intake in the micefed on the low energy density food increased at peaklactation on average of 3.8 g (from 23.1 to 26.9 g perday). Similar data are available for brown hares(Lepus europaeus; Hacklander et al. 2002). When fedon a diet with lower energy content, asymptotic foodintake in late lactation increased from 230–250 to280–300 g per day. Consequently, milk productionwas stable across the dietary treatments at approxi-mately 35 g per day for females raising singleoffspring and 70 g per day for females raising twins.Conversely, when energy density was manipulated inthe opposite direction, there was no indication thatenergy intake was elevated. In another example, ratsfed a high energy density diet during lactationdecreased their food intake to sustain energy intakeconstant (Denis et al. 2003).

An important aspect of the peripheral limitation ideais that milk production levels remain constant acrossthe different manipulations—reflecting the fact thatmammary glands are working at capacity. Severalstudies have measured milk production and supportthis prediction. Drummond et al. (2000) studied milkproduction in rabbits (Oryctolagus cuniculus) andobserved that, following natural deaths of some off-spring, the flow of milk was unaltered. Fink et al.(2001) studied lactation in captive mink (Mustelavision) and showed that in mothers raising litters ofthree, six and nine offspring, milk production did notincrease when litters increased from six to nineoffspring. Rogowitz (1998) demonstrated in cottonrats that levels of milk production in rats at 21 and 88Cwere similar—consistent with the mammary glands


working at maximal capacity. However, other studieshave failed to find this consistency ( Johnson &Speakman 2001; Krol & Speakman 2003a,b;Krol et al. 2003a,b). We have now studied food intakeand milk production in mice at peak lactation at threedifferent temperatures: 30, 21 and 88C. As predicted bythe peripheral limitation hypothesis, food intakeincreased across these measurements as temperaturedeclined. However, unexpectedly, milk productionand, consequently, pup growth were not constantacross the different temperatures, but followed thepattern of food intake. Mice exported 88 kJ energy inmilk per day at 308C, 167 kJ at 218C and 288 kJ at 88C(Krol & Speakman 2003b). Moreover, the weaningmasses of pups at 30, 21 and 88C averaged 6.1, 7.0 and7.3 g, respectively (Krol & Speakman 2003a). Conse-quently, the colder it got the more food the mice ate,the more milk they produced and the heavier the pupswere at weaning.

These data are fundamentally inconsistent with boththe suggestion that the limits are imposed by thecapacity of the alimentary tract to process ingestedenergy and the suggestion that the limits reside in themilk production capacity of the mammary gland. Thereare a number of new ideas about the nature of the limitin food intake during lactation, which have beenrecently summarized (Speakman & Krol 2005). Forexample, one of these is that the limits are imposed byaspects of the neuroendocrinological system thatregulates food intake rates—the ‘neural saturationhypothesis’. An alternative, suggested by (Krol &Speakman 2003a,b), is that the capacity to expendenergy during lactation at 218C might be limited bythe ability of the female mouse to dissipate heat.Hence, manipulations at 218C—which aim to stimulateboth food intake and milk production—notablyincreasing litter size (Hammond & Diamond 1992),extending lactation (Hammond & Diamond 1994),making them simultaneously pregnant (Biggerstaff &Mann 1992; Koiter et al. 1999; Johnson et al. 2001b)and making them exercise (Perrigo 1987)—all failedto increase either food intake or milk production,because the animals could not increase their heatproduction without risking fatal hyperthermia (seebelow). In effect, this hypothesis is a central limitationidea, but is focused around the ability to dissipateheat rather than assimilation from the gut. Under theheat dissipation limit hypothesis, when mice areexposed to the cold, this is not an additional demand,but a relaxation of the heat dissipation limit, allowingthe animals to elevate not only their food intake but alsotheir milk production and thus the size of theiroffspring. Similarly, when mice were placed in thehot, this reduced their capacity to dissipate heat,restricted their food intake and milk production, andled to smaller pups being weaned.

There are two putative mechanisms for how thecapacity to dissipate heat may influence lactationperformance. At high ambient temperatures, lactatingmice may continuously face difficulties dissipatingheat. This would lead to a perpetually elevated bodytemperature (see below) which might influence theregulation of milk production. It is well establishedthat endogenous opioids in the preoptic anterior


hypothalamus are of key importance in the regulationof body temperature, and are reduced during lactation(Kim et al. 1997). Projections from this areaterminate in the paraventricular nucleus (PVN),where the magnocellular cells synthesize oxytocin.Rayner et al. (1988) showed that intracerebroventri-cular (ICV) administration of morphine inhibitsoxytocin production in the PVN. Thus, elevatedendogenous opioids under hyperthermia mightdirectly reduce oxytocin secretion. Morphine admin-istration also disrupts maternal suckling behaviour(Cox et al. 1976; Bridges & Grimm 1982). Inaddition, thyroid hormone, which may be regulatedby differences in body temperature, is an importantmodulator of prolactin production. Continualmaternal hyperthermia in relation to external ambienttemperatures (or differential capacity to dissipatebody heat) may therefore directly inhibit oxytocinand prolactin secretion, thereby reducing milk pro-duction. Finally, there may be a more direct linkbetween hyperthermia and milk production. Aresponse to hyperthermia might be to direct bloodflow away from the mammary glands to otherperipheral areas to dissipate heat by vasodilatation(Black et al. 1993). Blood flow in the mammaryglands has been shown to have a direct effect on milkproduction (Vernon & Flint 1983).

A second mechanism is that suckling schedules ofthe mice may be influenced by the development ofmaternal hyperthermia in the nest. Although pups actas heat sinks early in their development (Scribner &Wynne-Edwards 1994a,b), in late lactation offspringare capable of considerable heat production. Thesuckling unit of mother and pups, therefore, maygenerate heat that leads to maternal hyperthermia,ultimately forcing the female to discontinue suckling(Croskerry et al. 1978). The progress to hyperthermiawould be more rapid as the capacity to dissipate heatdeclines. The sucking stimulus is one of the primaryfactors stimulating oxytocin release and milk let down,and also feeds back onto prolactin release, therebyregulating milk production. Continual disruption ofsuckling, due to intermittent hyperthermia, would be asecond mechanism linking heat dissipation capacity tolactation performance.

Many studies have examined the pattern of changein food intake across the reproductive cycle amongother small non-domesticated mammals. Someexamples of these measurements are in table 1. Thisis not an exhaustive review. Using the data for rodentsand converting estimated food intakes into energy forthose species where only food intakes are reported inthe original papers, there was a significant effect ofbody mass (FZ42.6, p!.001) and a significant effectof reproductive status (FZ30.6, p!.001), but nosignificant interaction between these two variables(FZ1.55, pZ0.225; figure 2a). Using pairwise Tukeycomparisons, the non-breeding energy demands didnot differ significantly from those in pregnancy, but thedemands in lactation were significantly elevated aboveboth the non-breeding and pregnancy demands. Thisanalysis reveals that the energy costs of pregnancy arerelatively trivial. This does not mean that demandsin pregnancy do not impose limits on reproduction

Table 1. Some examples of peak energy intakes of non-domesticated small (less than 1 kg) mammals at different phases of thereproduction cycle. LS is litter size. Under P this is litter size at birth; under L, this is litter size at weaning. Where original paperdoes not state which, it is entered under both. Status: NB, non-breeding; P, pregnant; L, lactating. ! NB is the intake expressedas a multiple of the non-breeding intake. In all cases (except �), the animals were studied in the laboratory at standard roomtemperatures (21–248C) with ad libitum access to food. �� animals kept at 58C � animals kept at 108C. In some cases, the originaldata were quoted relative to body mass0.75, and in these instances the actual intakes have been recalculated using the cited bodymass. In other papers, only mass of food ingested is cited and these have been converted assuming a dry mass energy content forthe food of 20.9 kJ gK1, unless a different value was cited in the paper. In yet other studies, average daily metabolic rate (ADMR)is quoted. I have converted this to energy intake assuming a digestive efficiency of 80%. Genera: Rodents C. Z Clethrionomys,P. Z Peromyscus, M. Z Microtus, Me. Z Meriones, A. Z Acomys, Ca. Z Cavia, S. Z Sigmodon, Sc. Z Sciurus. Insectivores,S. Z Sorex, C. Z Crocidura, Ec. Z Echinops, E. Z Erinaceus. Bats P. Z Plecotus, T. Z Tadarida, E. Z Eptesicus, M. Z Myotis,R. Z Rousettus. ^ estimates relative to NB calculated using the NB in reference 7. $ estimates expressed as difference tounquoted NB level in original ms.

species mass status LS food intake energy (kJ dK1) ! NB refa

rodentsP. polionotus 13.4 NB 54.0 1

L 3.64 94.0 1.74 1C. gapperi 18.8 NB 3.80 2

34.0 P 5.58 6.39 1.68 227.0 L 12.0 3.15 2

P. maniculatus 14.5 NB 58.4 1L 4.30 144.6 2.47 1

P. maniculatus 20.1 NB 5.14 3.5 325.8 L — 9.6 2.70 3

P. leucopus 21.0 NB 72.45 1L 3.91 143.64 1.98 1

P. eremicus 21.5 NB 52.0 1L 2.42 104.9 2.02 1

M. pennsylvanicus 24.0 NB 4.82 238.2 P 5.05 6.57 1.36 231.0 L 15.5 3.21 2

C. glareolus 24.5 NB 73.2 4— P 5.0 99.2 1.35 4— L 4.0 162.8 2.22 4

P. leucopus 25.0 NB 4.0 3.2 5L 6.0 1.875 5

24.5 NB�� 4.0 5.5 5L�� 8.7 1.58 5

M. arvalis 25.3 NB — 62.3 633.9 P 4.25 66.5 1.07 6

— L 4.00 175.7 2.82 6P. leucopus 25.8 NB 62.8 7

25.8 L 5.0 8.23 150.1 2.39 7Phodopus sungorus 30.0 NB 56.0 8

41.0 P 5.6 68.0 1.21 835.0 L 104.3 1.93 8

P. floridanus 42.0 NB 94.5 1L 2.25 144.0 1.54 1

Onchomys leucogaster 45.3 NB 4.15 72.8 950.7 L 4.0 9.49 166.0 2.28 9

A. cahirinus 49.4 NB 45.8 1065.8 P 2.0 60.9 1.33 1046.1 L 2.0 63.1 1.38 10

M. brandtii 59.3 NB 94.54 1183.6 P 8.5 105.22 1.11 1151.5 L 8.5 334.2 3.53 11

Me. crassus 93.6 NB 113.1 1290.7 L 3.75 120.1 1.05 1288.8 NB 112.5 1290.6 L 4.0 121.5 1.08 1292.0 NB 147.9 1292.5 L 4.18 153.9 1.04 12

S. hispidus 125.8 NB 146.8 13180.2 P 5.0 233.9 1.60 13125.0 L 5.0 193.7 1.30 13

(Continued.)



Table 1 (Continued.)

species mass status LS food intake energy (kJ dK1) ! NB refa

S. hispidus 175 NB 158.5 14241 P 7.0 240 1.51 14169 L 6.9 450 2.84 14

S. hispidus 145.7 L� 5.0 390.4 2.65^ 15127.5 L 5.0 269.3 1.83^ 15

Ca. porcellus 402.6 P 1.7 C160$ 16550.5 L 1.7 C340 16615.8 P 2.5 C200 16628.8 L 2.5 C380 16

Sc. niger 880 NB 1516 17L 4.0 3579 2.36 17

primatesCallithrix jacchus 418.1 NB 34.58

P 45.71 1.27 18375.7 L 64.28 1.85 18

insectivoresS. minutus 4.9 NB 43.0 19

P 9 48.0 1.12 19L 9 173.0 4.02 19

Geogale aurita 5.9 NB 9.67 209.6 P 11.6 1.20 20

S. araneus 9.3 NB 61.8 2114.8 P 6.9 85.3 1.38 2110.7 L 4.3 175.4 2.83 21

S. coronatus 10.3 NB 46.6 19P 5.1 60.0 1.29 19L 5.1 191.0 4.09 19

C. russula 13.8 NB 45.3 19P 4.4 50.2 1.09 19L 4.4 128.0 2.82 19

C. viaria 17.1 NB 45.9 19P 3.5 48.5 1.06 19L 3.5 106 2.31 19

C. olivieri 39.5 NB 47.4 19P 3.8 47.5 1.00 19L 3.8 132 2.78 19

Ec. telfairi 174.8 NB 49.6 22279 P 1.5 121.0 2.44 22262.6 L 1.5 151.0 3.04 22

E. europaeus 750 NB 87.2 23850 L 3.0 187.5 2.15 23

batsM. lucifugus 9.0 P 1 33.7 24

6.7 L 1 60.3 1.79 24P. auritus 7.35 NB 1.8 48.0 25

8.53 L 1 2.0 53.0 1.10 25T. brasiliensis 13.4 P 57.0 26

11.4 L 1 114.0 2.0 26E. fuscus 20.84 P 1 48.9 2.15 27

17.4 L 1 105.1 2.15 27R. aegyptiacus NB 200.0 28

P 271.0 1.35 28L 360.0 1.80 28

marsupialsCaluromys philander 304 NB 2544 29

308 L 4124 1.62 29

a References: 1. Glazier (1985), 2. Innes & Millar (1981, 1985), 3. Millar (1985), Millar & Innes (1985), 4. Kaczmarski (1966), 5. Hammond &Kristan (2000), 6. Migula (1969), 7. Millar (1978), 8. Weiner (1987), 9. Sikes (1995) 27, 10. Degen et al. (2002), 11. Liu et al. (2003), 12. Kamet al. (2003), 13. Randolph et al. (1977), 14. Mattingly & McClure (1982), 15. Rogowitz (1998), 16. Kunkele (2000), 17 Havera (1979), 18Nievergelt & Martin (1999), 19. Genoud & Vogel (1990), 20. Stephenson & Racey (1993b), 21. Poppitt et al. (1993), 22. Poppitt et al. (1994),23. Krol (1985), 24. Kurta et al. (1989), 25. Mclean & Speakman (1999), 26. Kunz et al. (1995), 27. Kurta et al. (1990), 28. Korine et al. (2004),29. Atramentowicz (1992).


(see above arguments regarding foetal volume compet-ing for abdominal space with the alimentary tract).Nevertheless, the pattern observed in laboratory mice


and rats that the costs of reproduction are substantiallygreater during lactation than in pregnancy appears tobe very broadly applicable.

5.55.04.54.03.53.02.5

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Figure 2. (a) Peak energy intake of female small rodents,when not breeding (black), pregnant (red) and lactating(green). There was no significant elevation in intake duringpregnancy, but intake during lactation was significantlyelevated compared with both non-breeders and pregnantindividuals. (b) Residual energy intake in lactation wassignificantly associated with litter size. Data from smallrodents are given in table 1.


There was a wide variation in the energy demands at

peak lactation (table 1; figure 2a). The residual

variation, once the effects of body mass had been

removed, was significantly associated with litter size

(figure 2b), which explained 49.3% of the residual

variance ( pZ0.0103). Although litter size and body

size explain much of the variation in energy intake at

peak lactation, there is still considerable variation in the

energy intakes across species. One factor that may be of

importance in this variation is the developmental

strategy. Studies of the patterns of development in

eutherian mammals have identified two separate

developmental strategies (reviewed in Martin &

MacLarnon 1985; Martin 1989), with some species

having prolonged periods of gestation followed by birth

of relatively well-developed offspring that rapidly

become independent of the mother and are capable

of feeding themselves relatively quickly after birth

(precocial development). In contrast, other species

have a relatively short gestation that is followed by a

more protracted period of lactation during which they

are completely dependent on nutrient supply from

the mother (altricial development). The use of these

strategies is not independent of body mass with


mammals weighing more than 100 kg using only theprecocial strategy, while species weighing less than100 g use the altricial strategy almost exclusively,although some species follow what has been calledan ‘intermediate’ strategy (defined in Martin &MacLarnon 1985), e.g. Acomys caharinus (Degenet al. 2004). As anticipated from their size, mice followthe altricial strategy. Between 100 g and 100 kg speciesare found that follow either precocial or altricialstrategies with a few ‘intermediates’ (Martin &MacLarnon 1985; Martin 1989).

Clearly, this dichotomy in developmental strategymay impact on the levels of energy investment by themother during lactation (Oftedal 1984; Hill 1992; Kamet al. 2006). Since it is the rarer strategy, relatively fewstudies have addressed the levels of food intake of smallrodents following the clearly precocial strategy (but see,for example, the studies of the guinea pig Caviaporcellus; Kunkele & Trillmich 1997) with many morestudies following the patterns of investment duringlactation of species raising clearly altricial or inter-mediate offspring. Gestation period (days) reflectingposition on the altricial–precocial continuum was notan additional significant factor influencing peak energydemands ( pZ0.13). The absence of an effect of thisdichotomy might be expected given the selection ofspecies that were either altricial or intermediatestrategists. The only rodent with clear precocialdevelopment in the data reviewed in table 1 was theguinea pig (Krebs 1950), but the manner in which thedata were presented in that paper relative to unstatedlevels in non-breeding animals precluded its inclusionin the data analysis.

The residual variation once the effects of litter sizeand body mass were removed to an extent followdifferent strategies for coping with the direct costs ofreproduction. Hence, the low values in the cotton rat(Sigmodon hispidus) and the hamster (Phodopussungorus) reflect the fact that these animals deposit fatstores during the early phase of reproduction, which arewithdrawn in lactation and reduce the need to supply allthe energy from food intake. Low demands in otherspecies may reflect the use of compensatory mechanismsthat reduce expenditure on other components. Thesestrategies will be discussed in detail below.

Expressing the peak lactation energy intake as amultiple of the intake of non-breeding animals, theaverage across all the rodents in table 1 was 2.1 (s.d. Z0.72, nZ21). This is considerably lower than the ratioin MF1 mice at 218C, which equalled 4.2. However,the intake of the MF1 mice at this temperature iswithin the 95% confidence limits of the predictionfrom the fitted regression equation from the non-domesticated rodents, given the body size and littersize for this animal.

The importance of the differences in magnitudebetween the peak levels of intake in wild rodents andlaboratory mice and rats remain uncertain. In most ofthe species studied in table 1, and in other species, peakintake of food during lactation is dependent on littersize. Generally, however, these patterns do not reach anasymptotic level as observed in the MF1 mice(figure 1b). Two examples are shown in figure 3.However, despite this absence of an asymptote in both

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Figure 3. (a) Food intake at peak lactation (grams over3 days) in relation to litter size in Peromyscus leucopus (drawnfrom tabulated data in Millar 1978). (b) Energy intake duringlactation (kJ dK1) in relation to litter size in Sigmodon hispidus(drawn from tabulated data in Rogowitz 1998).

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Figure 4. Mean mass at weaning of male (grey) and female(white) pups of (a) rats and (b) mice when fed on diets ofvarying protein content (from 16.3% to 9.4% protein).Drawn from data in Goettsch (1960).


species (and others), there is still an inverse relationship

between the litter size and the mean offspring size,

indicating that increases in investment were insufficient

to match elevated demand. These data suggest that the

trade-off between litter size and pup mass is more

complex than the simple model of the mother reaching

an asymptotic intake at which point a fixed investment

is divided between increasing numbers of offspring.

Part of this equation, however, may be capacity limits

in the offspring themselves and how these depend on

litter size. Pup–pup competition may be a key element

of the impact of litter size on their growth efficiency.

(ii) Protein and calciumPrevious discussion of limits on lactational per-

formance has focused almost entirely on energy. Yet

growing offspring also require large amounts of protein,

calcium and other micronutrients. At weaning, an MF1

mouse has produced on average 12 pups each weighing

7.5 g: a total of 96 g of wet tissue. Of this tissue,

approximately 13.5 g is protein and 1.8 g is calcium,

both of which must be supplied by the mother. The diet

we feed our mice on contains 22.45% crude protein

(RM3 breeder diet SDS diet services, Witham Essex).

Thus, over the last 8 days of lactation when they are

taking in 23.2 g each day, their total protein intake

amounts to 41.6 g of protein—substantially more than

ultimately appears in the pups. This rough balance

suggests that in the lactating MF1 mouse system,

protein supply is unlikely to exert limits on the animals.


Several studies have been performed, however, where

the level of protein in the diet is much lower.

Goettsch (1960) examined the effects of feeding

nursing rats and mice on four diets containing 16.3,

13.6, 11.4 and 9.4% protein. Food intake in lactation

was broadly independent of the protein content of the

diets, except that it was reduced in mice feeding on the

lowest protein diet and ‘highly variable’ in rats on this

diet. However, the growth of offspring was directly

affected in both rats and mice, with offspring size at

weaning being positively related to the protein content

of the diet (figure 4a,b). Maternal mean protein intake

in these mice throughout lactation averaged only 0.5 g

per day in the lowest protein group which even

accounting for the mean litter size difference between

the strains (12 in our work and 7.1–7.5 in Goettsch

1960) is still substantially lower than that for our mice

(5.2 g/day at peak lactation). There are several

additional studies which show that low maternal

dietary protein levels affect pup growth. The mothers

appear constrained, however, owing to the maximal

amounts of energy that they can process—perhaps

defined by the heat dissipation limit. Hence, this

energetic limit prevents them from simply eating

more food to meet their protein demands. These data

indicate that energy limitation imposes a primary

constraint on the animals under low protein supply.

If low protein in the maternal diet imposes a restriction

on the growth of offspring then, perhaps, protein content

of the diet is the key factor that links food intake to pup

growth. When we exposed lactating females to hot and

cold conditions, we may have manipulated the constraint

on their total energy intake that was imposed by their

capacity to dissipate heat (Krol & Speakman 2003a,b).However, the impact on pup growth may have been


mediated via the consequences of these different levels ofenergy intake for levels of protein intake—indeed themice at 88C were ingesting approximately 7.2 g of proteineach day, those at 218C only 5.2 g per day and those at308C only 2.91 g per day. Perhaps the trade-off betweenlitter size and pup size (figure 1c) may move depending ondietary protein contents—to the right when protein in thediet is elevated and to the left when it is reduced.Supporting these ideas, Hitchcock (1927) fed nursingrats a base diet that contained 29% protein, andadditionally supplemented some of the animals withraw meat. Litter size in both groups was the same yet theoffspring from the mothers fed meat daily weighed 61.8 gat weaning, compared with only 47.3 g in the rats notgiven the meat ration.

A similar situation probably pertains to calciumsupply. The diet we use in our studies of mouselactation contains 1.24% Ca. Thus, in the last 8 days oflactation, the mice ingest 2.30 g of calcium, comparedwith an estimated calcium content of the pups of 1.8 g.Hence, as long as calcium and protein levels in diets areabove a certain limit, the constraint on maximal energyintake is unlikely to severely impact offspring in termsof their protein and calcium status.

For wild animals, the situation where calcium andprotein contents of the diet are below the critical point,where the energy limit becomes restrictive, may bemore routinely breached. One situation that hasattracted considerable attention is in microchiropteranbats. Because microchiropteran bats are almostexclusively nocturnal, a constraint probably imposedby ecological factors such as predation risk andcompetition (Speakman 1990; 1991a,b; Rydell &Speakman 1995; Speakman et al. 2000), they do notroutinely get any exposure to sunlight. This mayinfluence their production of vitamin D (Cavaleroset al. 2003), which is an essential component of calciumphysiology. Moreover, most microchiropteran speciesare predominantly insectivores and these prey are verylow in calcium content. Their inability to manoeuvreeffectively on the ground restricts them from findingadditional mineral sources of calcium that may beavailable to other animals. Insectivorous bats have verylow litter sizes for their body sizes (Kurta & Kunz1987) and one interpretation of these low rates ofproductivity is that they are primarily constrained bytheir capacity to obtain calcium (Barclay 1994).

(b) Organ remodelling to meet these demands

The alimentary tract has a more limited capacity toprocess energy than the mouth has capability to supplyit. This is why the gut includes storage organs like thecrop and stomach at the entry to the tract to take onboard the intake at this elevated rate. If an animalrequires to increase its food intake from approximately5 g per day, prior to breeding, to 23 g per day at peaklactation, it could probably achieve this intake immedi-ately, at the level of the mouth. There are no obviouschanges in the morphology of the mouth as the animalsprogress through reproduction. However, while a non-breeding mouse might be capable of eating 23 g eachday, its alimentary tract would probably be unable toprocess the intake. Hence, animals need to modify theirinternal architecture to cope with the altered demands.


That lactating animals modify their tracts and associ-ated organs in this manner has been known for at least45 years. During lactation, there is an increase in thesizes of both the liver (Kennedy et al. 1958) and thepancreas (Jolicoeur et al. 1980). The most dramaticchanges however are in the alimentary tract itself,which involve major morphological increases in theabsorptive surface of the intestinal mucosa and alsogrowth in the length of the tract (Boyne et al. 1966; Fellet al. 1963; Campbell & Fell 1964; Craft 1970;Cripps & Williams 1975; Burdett & Reek 1979; Prietoet al. 1994; Speakman & McQueenie 1996; Hammond1997). These changes are paralleled by alterations inthe transport capacity at the cellular level (Larradaleet al. 1966; Dugas et al. 1970). The growth of themucosal layer of the alimentary tract in lactationincludes both hypertrophic (cell expansion) andhyperplastic (cell proliferation) responses (Fell et al.1963; Cairnie & Bentley 1967; Prieto et al. 1994).These changes effectively allow the lactating female tocontinue to extract the same amount (%) of energyfrom the ingested food independent of the rate of intake(e.g. Campbell & Fell 1964; Hammond et al. 1994).

Studies of wild rodents reveal similar patterns ofchange, although generally including more modestincreases (e.g. Myrcha 1964; Chlethrionomys glareolus;Myrcha 1965 Apodemus flavicollis; Gebczynska &Gebczynski 1971 Clethrionomys oeconomus) reflectiveof the lower level of increase in food intake betweennon-breeding and lactation states (table 1). This isconsistent with the fact that the extent of modificationof the intestine is related to inter-individual variationsin litter size in mice.

Several hypotheses have been advanced about howthe growth in the gut is stimulated. The first is that thehormones that underpin the increase in food intakeduring lactation (reviewed in Speakman & Krol 2005),including elevated prolactin and reduced leptin,directly stimulate the changes in the alimentary organs.Alternatively, the food intake itself may result in theproduction of local growth factors stimulating tissueproliferation (Datta et al. 1995). Finally, hormoneslinked to milk production, e.g. oxytocin, may stimulatethe growth. Unfortunately, separating these effects isdifficult. Injection of hormones, like prolactin, affectfood intake (Noel & Woodside 1993), so eliminating asecondary local effect stimulated by the elevated foodintake would need animals to be pair-fed with non-injected controls, and these critical experiments havenot yet been performed. Hammond (1997), however,noted that hypertrophy of the gut is a generalizedresponse to elevated nutritional demands (such as coldexposure) and the hormonal profile in these circum-stances is completely different from that in lactation,favouring the hypothesis that changes are primarilystimulated by a local response, stimulated directly bythe food intake.

Many other morphological changes occur duringpregnancy and lactation, not least of which is thegrowth of mammary tissue during late pregnancy tofacilitate milk production during the subsequentlactation. As might be expected, considerable attentionhas been paid to this growth process and its hormonalbasis, particularly the roles of sex steroids progesterone


and 17b-oestradiol (reviewed in Lamote et al. 2004).Not all changes in the lactating animal, however,include expansion of tissue sizes. In particular, thereis a large reduction in the size of the adipose tissuestores (Speakman & McQueenie 1996; Vernon & Pond1997). These reductions in the size of adipose tissuestores have generally been interpreted as withdrawal ofstored energy to support energy delivery during thelactation event. Certainly, in some species (like thecotton rat: Randolph et al. 1977 and the Siberianhamster: Weiner 1987), there is accumulation of fatduring pregnancy that is withdrawn later in lactationand this strategy appears to reduce the peak food intakedemands in lactation (see above and table 1). However,in other animals, the contribution of fat to the overallenergy budget is relatively trivial. In mice, for example,the fat stores decline by approximately 2 g duringlactation (Johnson et al. 2001c) equivalent to approxi-mately 80 kJ of energy, compared with a metabolizableenergy intake over the last 10 days of lactation ofapproximately 2600 kJ.

The increasing recognition that adipose tissue is notonly an energy store but also an endocrine regulatorsuggests an alternative explanation of the reduction infat mass may be that this reduces production of leptin(and possibly other adipokines) which then act tostimulate food intake. That low levels of leptin duringlactation are involved in stimulating food intake hasbeen demonstrated by repleting leptin levels usingminiosmotic pumps. This provision of exogenousleptin blunts the level of increase in food intake duringlactation (Stocker et al. 2004). Decreased leptin levelsmay also be important in signalling reductions in theUCP-1 gene expression in brown adipose tissue oflactating rodents, the significance of which will beaddressed below. Elevated levels of circulating adipo-kines are often inferred to underlie the links betweenobesity and disease. For example, high leptin levels,independent of adiposity, are an increased risk factorfor the development of the metabolic syndrome inhumans (Franks et al. 2005). By reducing the levels ofproduction of these compounds, lactating animals mayactually derive an advantage relative to non-lactatingindividuals.

It is often suggested that a direct impact of thisremodelling of the internal architecture during lacta-tion is an increase in the resting metabolic rate relativeto non-breeding animals. This occurs in theory becausethe animal expands the size of tissues that have highrates of metabolism (e.g. the liver and alimentary tract;Field et al. 1939; Krebs 1950), while simultaneouslydecreasing the size of other tissues with low energydemands. This is the direct energy cost linked to thelactation process of making these morphologicalmodifications. When comparisons are made of theresting or basal energy expenditure in lactating smallmammals compared with non-breeding individuals,many studies have reported that the RMR of lactatinganimals is higher (Camas et al. 1982; Stephenson &Racey 1993a,b; Garton et al. 1994; Poppitt et al. 1994;Harder et al. 1996; Speakman & McQueenie 1996;McLean & Speakman 2000; Johnson et al. 2001b;Zenuto et al. 2002; Krol et al. 2003a,b). Although otherstudies, particularly in non-domestic species, have


reported minor or even no changes (e.g. Dryden et al.1974; Turbill & Geiser 2006), this might be anticipatedin those groups where the increase in food intake inlactation is more modest (table 1) and thus requirescorrespondingly minor adjustments in morphology.However, if the theoretical basis of the effect is thatorgans such as the alimentary tract and liver requiregreater maintenance costs (Field et al. 1939, Krebs1950), then one would anticipate that differencesbetween lactating individuals in the sizes of theseorgans would have impacts on metabolism, andcorresponding impacts on food intake and lactationperformance. However, when such associations havebeen sought, they have generally not been found(reviewed in Speakman et al. 2004). It remainsuncertain why these expected trends are not observed,but this lack of an association calls into question thecost of the tissue remodelling.

3. INDIRECT COSTS(a) Optional compensatory costs

(i) Thermoregulatory demandsDuring lactation, many species of small mammalsexperience large morphological and biochemicalchanges in their interscapular brown adipose tissue(BAT). These modifications include reductions in theamount of BAT in mice, rats, ground squirrels andhamsters (Agius & Williamson 1980; Wade et al. 1986Johnson et al. 2001b). In addition to reductions inoverall tissue mass, there are also reductions in BATmitochondrial mass (Trayhurn et al. 1982; Trayhurn &Jennings 1987a,b). In late lactation, mitochondria-specific content of uncoupling protein 1 (UCP-1) isreduced to only 8% of the level found in non-breedingmice (Trayhurn & Jennings 1987a,b, 1988) and to 26%in ground squirrels (Nizielski et al. 1993). GDPbinding, which is a measure of mitochondrial thermo-genic capacity, is also reduced in mice and rats(Trayhurn et al. 1982) and ground squirrels (Nizielskiet al. 1993), but not in hamsters (Wade et al. 1986).Brown adipose tissue is the key thermogenic organ insmall rodents (Cannon & Nedergaard 2004) and thechanges observed in lactating rodents mediate areduction in the noradrenaline-induced non-shiveringthermogenesis (Trayhurn et al. 1982; Trayhurn 1983),which is rapidly reversed upon weaning (Trayhurn &Jennings 1987a,b, 1988). In rats, the extent of decreasein thermogenic capacity is related to litter size (Isleret al. 1984), but this does not appear to be the case inmice (Trayhurn & Wusterman 1987a,b). Thesemorphological, physiological and biochemical changesin BATappear to be controlled by reduced sympatheticactivity in lactation (Trayhurn & Wusterman 1987a,b),which may be responsive to elevated corticosteroidlevels (Vernon & Flint 1983). Treatment of lactatingrats with exogenous leptin completely reversed thedownregulation of UCP-1 gene expression in BAT(Xiao et al. 2004). Changes in other aspects of BATphysiology are also apparent during lactation, includingreduction in the activity of iodothyronine 5’-deiodi-nase, which catalyses conversion of thyroxine (T4) totriiodothyronine (Giralt et al. 1986). All these changesare consistent with small lactating animals attempting


to reduce obligatory heat production from BAT.Trayhurn (1989) interpreted this reduction as anenergy-saving mechanism that increased the efficiencyof milk production. However, in the light of our studiesof limits to food intake in lactating mice describedabove, an alternative interpretation is that this down-regulation does not save energy which can be used formilk production, but rather reduces the heat burden onthe animal, allowing it to elevate milk production.

In recent years, a number of additional uncouplingproteins have been described (UCP-2 to UCP-5).These uncoupling proteins have different tissue distri-butions, with UCP-2 being very widespread, UCP-3being restricted to BAT and skeletal muscle, and bothUCP-4 and UCP-5 being found only in the brain. Therole of these UCPs in resting and thermogenicheat production has been an issue of debate (e.g.Erlanson-Albertsson 2002, 2003). Studies of the UCP-1 knockout mouse indicate that the other UCPs cannotreverse the lack of thermogenic capacity brought aboutby the absence of UCP-1 (Enerback et al. 1997;Golozoubova et al. 2001; Nedergaard et al. 2001).Although UCP-3 cannot be facultatively upregulatedto replace the function of UCP-1, when it istransgenically overexpressed, the resultant mice haveelevated resting metabolism (Clapham et al. 2000).Surprisingly, given the suggested absence of any rolefor natural levels of UCP-3 in thermogenesis, it hasrecently been shown that UCP-3 is also downregulatedenormously in BAT during lactation, and this isreflected in reduced protein levels as well (Pedrazaet al. 2000, 2001; Xiao et al. 2004), but UCP-2 isunchanged (Pedraza et al. 2001). Levels of UCP-3 geneexpression in muscle are also decreased in lactation(Xiao et al. 2004). These effects appear to reflectcirculating levels of free fatty acids (Pedraza et al.2000). Possibly, UCP-3 is thermogenically insignif-icant but it is incidentally downregulated because themechanisms for regulating UCP-1 and UCP-3 aresimilar. There may be consequences of this down-regulation, which the animal cannot avoid (Cadenaset al. 2002), and will be discussed below under‘consequential costs’.

Many small mammals are able to make profoundsavings of energy by relaxing thermoregulation atnormothermic levels and entering periods of torpor,during which body temperature is commonly reducedto levels just above ambient (Heldmaier et al. 2004).While bringing significant benefits in terms of energysaving, torpor is fundamentally incompatible withsome of the processes of reproduction as was firstdemonstrated in a series of elegant experiments in theearly 1970s, where bats were forced into torpor forvarying periods with the consequence that gestationperiod was extended by the exact same period that themice had been made to spend in torpor (Racey 1973).Foetal growth therefore halts when bats enter torpor.The significance of this effect was demonstrated in wildpopulations of bats during the early 1980s, when it wasshown that adverse weather conditions during earlypregnancy slowed foetal growth in wild bat populationsresulting in significant year-to-year differences in theduration of pregnancy (Racey & Swift 1981). Thesefindings seem to be at odds with the fact that


hibernating bears gestate and lactate. However, studies

of body temperature in bears during lactation revealthat while metabolism is downregulated to a similar

extent as in small mammals, body temperatures only

cool to approximately 30–328C. If the interruption offoetal growth is temperature mediated, this difference

in body temperatures during torpor may explain howbears are able to combine their reproductive functions

with hibernation. Despite these apparently negativeimpacts of torpor in pregnancy, it is used frequently in

some species of marsupial (e.g. dunnarts (Sminthopsismacroura): Geiser et al. 2005; and mulgaras (Dasycercuscristicauda): Geiser & Masters 1994), although in these

animals the body temperature is still defended atapproximately 148C.

The impact of torpor on lactation is less clear. Many

species appear to use periodic entry into torpor as anenergy-saving mechanism during lactation (Geiser

1994; Turbill et al. 2003) and energy budgetingindicates that this effect may be sufficiently great that

the species concerned only need to make minorincreases in food intake during lactation to achieve an

overall energy balance. For example, the mean dry food

consumption of non-reproductive brown long-earedbats averaged 1.8 g/day (48 kJ dK1) while lactating bats

ate 2.0 g each day (53 kJ), yet the bats in lactation wereable to export 22 kJ dK1 of this intake as milk

(Mclean & Speakman 1999) while deriving only on

average 1.2 kJ from stored fat. Physiological compen-sation of energy budgets in this manner, so that food

intake requirements are unaltered, may clearly haveprofound effects on the ecological costs of reproduc-

tion. Brown long-eared bats in the wild do not increasetheir flight times between pregnancy and lactation

(Entwistle et al. 1996), a pattern repeated in other

species such as the common pipistrelle (Pipistrelluspipistrellus (Zpygmaeus); Swift 1980) and long-tailed

bats (Chalinolobus tuberculatus; O’Donnell 2002). Ifpredation risk during reproduction is a simple function

of time spent flying, then for these animals there may be

no ecological cost at all. This pattern however may bepeculiar to bats under particular energy stress (e.g.

Turbill et al. 2003) or at the margins of theirdistributions (Speakman et al. 1991), since studies of

other bat species more centrally in their distributionsyield different patterns. Big brown bats (Eptesicusfuscus) and little brown bats (Myotis lucifugus), for

example, have increased food intake during lactation(Kurta et al. 1989, 1990) and a decreased tendency to

enter torpor during lactation (Audet & Fenton 1988;Hamilton & Barclay 1994; Grinevitch et al. 1995;

Lausen & Barclay 2003). Flight time in northern bats

in Sweden is almost doubled in lactation comparedwith non-breeding individuals (Rydell 1993).

The fact that not all bats use torpor during lactationsuggests that there are significant costs associated with its

use. One possibility is that torpor is incompatible with

milk synthesis. Studies using explants of mammary tissuefrom bats confirm that they have no special protection

from reduced milk synthesis as temperature declinescompared with explants of rodent mammary tissue

(Wilde et al. 1999). Using torpor in lactation by bats atthe margins of their distributions may be a necessity

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Figure 5. (a) Food intake and (b) wheel-running activity ofrats during pregnancy and lactation. Days are expressedrelative to the day of parturition (day Z0) also indicated bythe dotted line. Data are split between those raising large(nO6; open squares) and those raising small (n!6; filledsquares) litters. Note how food intake in the larger littersincreases dramatically a few days before weaning probablydue to intake by the offspring (plots from data presented intables 2 and 3 of Slonaker 1924).


forced on them by the short duration of the night whichconstrains their capacity to extend flight times.

In spite of the energetic benefits and widespread useof torpor by bats in lactation, some groups that areotherwise capable of torpor appear to avoid using itduring lactation. For example, longitudinal records ofbody temperature in free-ranging female short-beakedechidna (Tachyglossus aculeatus) indicate that theydefend a continuous but low body temperaturethroughout the summer unless they lose their offspring(Nicol et al. 2005). Although many species of rodentare known to hibernate and use daily torpor, no speciesof rodent has ever been reported to use torpor duringlactation. Whether torpor and lactation are mutuallyincompatible in rodents was addressed by Stamperet al. (1998) who administered 2-deoxy-D-glucose(2-DG), a glucose analogue that interferes with cellularglycolysis, to lactating and non-lactating Siberianhamsters. 2-DG induced torpor in both groups,although the duration of torpor tended to be shorterin lactating animals. Evidence suggested that pups wereable to obtain milk from torpid mothers, but whetherany milk synthesis occurred was uncertain. Althoughapparently physiologically capable of entering torporfemales that were subjected either to a combination ofbrief food deprivation and food restriction or just foodrestriction failed to display torpor, but instead killedone or more of their pups (Stamper et al. 1998).

(ii) Physical activityIn a classic paper, Slonaker (1924) detailed thechanges in physical activity that occurred in female


rats during reproduction. Plots of some of the

meticulously reported data are shown in figure 5.These data show that rats have similar patterns of

food intake to mice during pregnancy and lactation,and that wheel-running activity is dramatically

suppressed during lactation to approximately one-third to one-half of that in pregnancy. The suppres-

sion of activity however was not proportional to littersize—since rats with small litters (n!6 offspring) ran

about the same amount as those with large litters(nO6 offspring) (figure 5). Mice (MF1) also show

similar reductions in spontaneous physical activityduring lactation (Speakman et al. 2001). Rabbits also

increase the time spent resting as lactation progresses

(Fernandez-Carmona et al. 2005).Equivalent data on physical activity for non-

domestic animals are scarce. This is probably becausemost activity in wild animals concerns foraging activity,

which is generally increased during lactation owing tothe elevated food intake requirements (see above). In

domestic animals, physical activity and food intake arenot inescapably linked, so modulations of activity

without affecting food intake are feasible. However,animals engage in other activities that are not

associated with foraging, e.g. grooming. We havemonitored the behaviour of bats during lactation and

found that brown long-eared bats (Plecotus auritus)significantly reduce the amount of time spent grooming

when compared with non-breeding females occupyingthe same roost. On average non-lactating bats were

observed grooming in 26.9% of the behaviour records,while lactating bats were only observed grooming in

11.1% of records. Similar high levels of time spentgrooming in non-lactating bats have been observed in

some other species (Shen & Lee 2000; Fleming et al.1998). However, such high levels are not universal(Winchell & Kunz 1996) and reductions in grooming

during lactation are also not observed in these species(Winchell & Kunz 1996). Female long-eared bats

groom their offspring during lactation but this timeallocation was relatively small and even when this was

taken into account, the lactating females we studied stillspent less than half of the time spent grooming by non-

breeding individuals (McLean & Speakman 1997).Grooming behaviour by bats is energetically expensive

(Giorgi et al. 2001) and plays a role in the removal ofectoparasites. By reducing the time spent grooming,

lactating bats may release significant amounts of energyfor lactation, but may pay a price in terms of elevated

ectoparasite burden. In the wild, in the mouse-earedbat (Myotis myotis), ectoparasite burden increases

during pregnancy and lactation (Christe et al. 2000)when it is also related to body mass—suggesting a role

for energy balance in driving ectoparasite burden.

However, potential interactions here are complex.Neuhaus (2003) experimentally removed ectoparasites

(mainly fleas) from female Columbian ground squirrelsusing a commercially available insecticide. Removing

parasites led to an increase in female body conditionduring lactation and at weaning and an increase in

weaned litter size. Hence, removing ectoparasites maybe costly, but not removing them may impose other

costs (see also Khokhlova et al. 2002).

35.536.036.537.037.538.0

38.539.039.5

06:00 12:00 18:00 00:0000:00time of day

body

tem

pera

ture

(˚C

)

Figure 6. Body temperatures of a single female MF1 mousemeasured at 1-minute intervals using an implanted bodytemperature transmitter (e-mitter Minimitter Inc) for 7 daysprior to breeding and for 7 days at peak lactation. Data arescreened to include only times when the animals are at rest,and then averaged over 30-minute periods within each day.The error bars refer to the s.d. for day-to-day variability (nZ7for each data set). During lactation, the body temperature isconsistently elevated approximately 1.58C higher than thatprior to breeding during the light phase and much of the darkphase. Dark bar indicates period when lights out ( J. R.Speakman, K. Drerrer, F. J. Munro & C. T. Troup 2005,unpublished data).


(iii) Immune systemThe adaptive interplay between immune function andreproduction has been extensively studied in birds(Cichon et al. 2001; Martin et al. 2008). Relatively littleattention has been paid to this trade-off in smallmammals. There is some evidence that particularlyenergetically costly phases of reproduction in smallmammals lead to impaired immune function. Thiseffect can be magnified if the reproductive eventcoincides with a period of food shortage. If rats areexperimentally malnourished, for example, duringlactation there is a marked increase in the numbers ofbinucleate lymphocytes in the maternal spleen (Ortizet al. 1995), increasing from 11.5% of cells in non-lactating controls to 21.5% in the malnourishedlactating animals. Since the spleen is an importantlymphopoetic organ in rodents, this direct cell damageduring energy restriction in lactation may haveimportant negative impacts on the mother’s immunesystem. Direct evidence of this impact of restriction wasthe experimental observation that malnourished rats inlactation had an impaired response to cholera toxin(Flo et al. 1994).

Even during normal pregnancy, there are significanteffects on the immune system. Committed B-lympho-cyte precursors in the bone marrow of pregnant micedecline by 6.5 days into the pregnancy and byparturition they are down to only 10% of control levels(Medina et al. 1993). Newly formed B cells were alsoreduced, but cells carrying myeloid and erythroidmarkers were not. This indicates the production andexport of B cells during pregnancy is much reduced.These effects could be mimicked by oestrogen treat-ment—suggesting an endocrine sex steroid-basedmechanism of control (Medina et al. 1993).

However, the reasons for this effect are unclear,because the energy demands of maintaining theimmune system are generally regarded to be trivial(Derting & Compton 2003). One possibility is that thereduction in body fatness at peak lactation itselfmediates reduced immunity, since studies in volesand hamsters have shown that surgical removal of bodyfat decreases immune capability (Demas et al. 2003;Demas 2004). A mechanism for this effect mightinvolve changes in circulating levels of leptin, as it hasdirect effects on immune cells stimulating T-cellimmunity, phagocytosis, cytokine production andhaemopoiesis, resulting in attenuated susceptibility toinfectious insults (Ingvartsen & Boisclair 2001). In thatcase, the immunocompetence cost of reproduction maynot be a compensatory cost but an obligatoryconsequential cost of the reduced fat content, whichmay itself be part of the mechanism of endocrinecontrol of lactational food intake or manipulation ofheat dissipation limits by effects on UCPs (see above).

(b) Obligatory consequential costs

(iii) HyperthermiaIt has been known for at least 50 years that high ambienttemperatures and humidity, or exposure to solarradiation, all negatively affect milk production in dairycows (Bos taurus; Cobble & Herman 1951; Brody et al.1958). Similar negative effects of high ambienttemperatures have been reported in other large domestic


animals such as pigs (Sus scrofa: Black et al. 1993;Quiniou & Noblet 1999; Renaudeau & Noblet 2001;Renaudeau et al. 2003) and sheep (Ovis aries: Abdallaet al. 1993). Many of these larger animals also exhibitchronic hyperthermia during lactation (e.g. sows,Ulmershakibaei & Plonait 1992). The risk of hyperther-mia in lactation therefore appears to be a significantobligatory cost in larger mammals. Heat dissipationdifficulties found in large domestic animals may be farless significant in small mammals owing to their morefavourable surface-to-volume ratios. Nevertheless,direct measurements of maternal body temperatures(figure 6) confirm that lactating mice are continuouslyhotter than their non-reproducing equivalents. Similareffects have been observed in rats (Croskerry et al. 1978;Leon et al. 1978, 1985; Kittrell & Satinoff 1988) andSiberian hamsters Phodopus sungorus (Scribner &Wynne Edwards 1994a). Rabbits also show negativeeffects of high temperatures on food intake and pupgrowth (Marai et al. 2001).

Studies of blood flow during lactation indicate thatthere is no major redirection of blood away from themammary glands to facilitate heat loss (cattle, Loughet al. 1990; goats Capra hircus, Sano et al. 1985; rabbits,Lublin & Wolfenson 1996). There is a substantial bodyof behavioural literature following the pioneering workof Leon et al. (1978), which suggests that sucklingbehaviour of small mammals may be influenced by therisks of maternal hyperthermia. These studies includedobservations that rats terminated suckling bouts whentheir body temperatures rose. Similar effects have beenreported in larger lactating animals that have contactwith their offspring such as sows (Renaudeau & Noblet2001), where elevated temperature led to reduceddurations and greater frequencies of suckling boutscoupled with lowered milk production and pigletgrowth. Perhaps the best evidence comes from directexperimental manipulations of body temperature andexamination of the subsequent effects on suckling


behaviour. These experiments have only been per-formed on rats, but have involved two separatemanipulations. Direct heating of the preoptic area ofthe brain, which led to premature termination ofsuckling bouts (Woodside & Leon 1980), and injectionof rats with morphine elevates body temperature andcauses disruption of maternal suckling behaviour(Bridges & Grimm 1982). However, rats are an orderof magnitude larger than mice, so perhaps of greaterrelevance are the studies of Siberian hamsters, whichshow that during the daytime, the time spent with thelitter in late lactation may be constrained by ambienttemperature (Scribner & Wynne Edwards 1994b).Interestingly, the two subspecies of Siberian hamster(P. sungorus sungorus and P. sungorus campbelli ) differ inthe rates at which the offspring develop their ownthermoregulatory capacities—faster in P. s. campbelli(Newkirk et al. 1998). This difference is reflected ingreater and earlier problems in maintaining bodytemperature during suckling bouts by female P. s.campbelli (Scribner & Wynne Edwards 1994b) andconsequent negative effects on pup growth in thissubspecies (Newkirk et al. 1998).

Despite a large literature indicating that there aredirect effects of the litter on maternal hyperthermia,there are some contradictory data. For example, thebody temperatures at which rats discontinue sucklingare generally lower than the levels they tolerate whileexercising outside the nest (Kittrell & Satinoff 1988).Perhaps more important are experimental inductionsof reduced body temperature using sodium salicylate,which did not extend suckling bouts (Bates et al. 1985).Although treatment of lactating rats with morphinesimultaneously elevates body temperature and disruptsmaternal suckling (above), the negative effect ofmorphine appears to be independent of its effects onbody temperature. This is shown by experiments inwhich morphine was administered with naloxone, anopioid receptor antagonist. Blocking morphine withnaloxone reversed the negative effects on maternalbehaviour (Bridges & Grimm 1982) but not maternalhyperthermia (Cox et al. 1976). Rats with experimen-tally induced increases in body temperature usingmorphine and naloxone combinations (Stern & Azzara2002) did not shorten their suckling bouts. However,this latter manipulation was performed only 7 days intolactation, and it would be instructive to know whether asimilar absence of the effects of combined morphine/naloxone treatment was apparent later in lactationwhen pup heat stress is more profound. Finally, rats fedlow-quality protein in their diets did not show reducedperformance when raised at 308C compared with 208C(Jansen & Binard 1991), even though their littersprobably generated similar heat dissipation problems.Overall, there is a definite negative effect of ambienttemperature on lactation performance of large animals,but the significance of these effects in smaller animalslike mice remains uncertain.

(iv) Disruption of sleep patternsAnyone who has had a child of their own is aware thatone of the key features of sharing ones life with a smallneonate is the disruption of adult sleep patterns toaccommodate suckling demands of the offspring


(Shinkoda et al. 1999). Only approximately 23% ofinfants sleep through entire nights (defined as from0000 to 0500 h) by eight weeks of age (Pinilla & Birch1993). Breast feeding mothers exhibit strongerdelta and theta power spectra during NREM sleep(Nishihara et al. 2004), but this pattern is the same onall nights whether sleep is disrupted or not to attend totheir offspring, suggesting that there is an associationbetween the sleep patterns and the hormonal changesin lactation. This is supported by observations thatslow wave sleep duration is approximately three timeslonger in females who breast feed compared with age-matched and body condition-matched individuals whobottle fed their offspring (Blyton et al. 2002). Effects ofdisruption of sleep patterns as a physiological cost oflactation in animals has been virtually unstudied. Inblack tufted ear marmosets (Callithrix kuhlii ), thepresence of young infants significantly impacted thesleep of mothers who were awake more than three timesas often as mothers without infants (Fite et al. 2003).Surprisingly, there was no impact on the sleep patternsof fathers—despite the fact that callitrichid mothersrelinquish the care of their offspring to fathers andalloparents very early in infant life. The impact of thedisruption of sleep on lactating female animalsis unknown.

(v) Bone lossDuring pregnancy and lactation, the female animalmust deliver sufficient calcium to her offspring toensure adequate bone development. During preg-nancy, females increase their absorption of calcium inthe alimentary tract (Halloran & DeLuca 1980).Although urinary losses also increase (Omi & Ezawa2001), the elevated absorption leads to elevatedcalcium levels in the blood (hypercalciuria), whichpersist throughout reproduction (Sarli et al. 2005). Inthe first trimester of pregnancy, this extra uptake mayactually exceed the foetal growth requirements. Con-sequently, there is a phase of bone accretion in theearliest part of pregnancy (Zeni et al. 1999; Omi &Ezawa 2001), which is characterized by increases inmarkers of bone turnover (like osteocalcin and alkalinephosphatase levels; Sengupta et al. 2005). This boneaccretion appears to act as a temporary storage ofcalcium that can be withdrawn as the pregnancyproceeds and the foetal demands for calcium increase.By the end of pregnancy, most studies reportsignificantly reduced bone mineral densities relativeto controls, although Leopold et al. (1995) indicatedthat levels were still elevated at day 20 in rats.

In lactation, the requirements for calcium are muchgreater, in line with the much larger offspring (as is alsothe case for energy and protein requirements).Lactating mothers reduce their levels of urinarycalcium excretion (Sarli et al. 2005) to facilitatesecretion into the milk. Although food intake inlactation is greatly elevated, lactating animals derive alarge amount of the calcium that is exported in milkfrom their own skeletons. This leads to a substantialreduction in the thickness of bones from across thewhole body (Miller & Bowman 2004). An additionaldirect effect of the calcium withdrawal from bone is areduction in dentine and enamel mineralization in the


teeth of lactating rats (Ozbek et al. 2004). On the face ofit, this use of skeletal calcium to provide therequirements for pup development may appear toindicate a shortfall in the available calcium from thediet, relative to the energy intake. However, threefactors suggest that this is not the case. First, simplebudgets of calcium availability relative to how muchappears in the offspring (see above) indicate that itshould not be limiting on most unsupplemented diets.Second, supplementing the dietary intake with calciumhas no impact on the extent of calcium withdrawal frombone (Weisstaub et al. 2003; Kovacs & Fuleihan 2006).Finally, in rats the extent of withdrawal is independentof litter size (at least comparing two to six pups), andhence neonatal calcium requirements (Sengupta et al.2005), although Tojo et al. (1998) did suggest that littersize correlated with the extent of withdrawal when datafor non-lactating animals (litter sizeZ0) were includedin the regression. In spite of this, the diet supple-mentation data strongly suggest that the loss of calciumfrom bone is not driven by a shortfall in dietary intake.

Although supplementing calcium in the diet doesnot impact on bone resorption, providing less dietarycalcium increases the rate of bone turnover (Gruber &Stover 1994). This is indicated indirectly by increasesin the markers of turnover (Sengupta et al. 2005) andalso directly from labelling studies. For example, bypre-labelling bone tissue of lactating female rats with45Ca, it was shown that under a calcium-deficient diet,40% of the label was lost from the whole skeleton overthe course of a normal pregnancy and lactation cycle—with losses in some bones like the femur being 75%(Wang et al. 1994). Under situations where the dietnaturally contains low calcium levels (e.g. fruit andinsects), lactating and pregnant individuals may seekout other sources to keep their calcium supply high(e.g. fruit bats, Ruby et al. 2000, Nelson et al. 2005 andinsectivorous bats, Adams et al. 2003).

A primary hypothesis therefore is that the endocrinemechanisms that promote the secretion of calcium intothe milk are incompatible with the retention of calcium inbone. Several lines of evidence support this hypothesis.First, prolactin, which is one of the key hormonesnecessary to support successful milk production in thelactating female, and the prolactin receptor, are alsofound in the synovial fluid (Ogueta et al. 2002). Studies ofthe impact of prolactin on mesenchymal stem cellsstrongly implicate it as playing a role in bone and cartilageformation and repair processes. Prolactin receptor KOmice (Kelly et al. 2001) have greatly elevated prolactinconcentrations and reduced bone formation. Prolactinsecretion to support lactation may therefore influencebone resorption.

Ovariectomized rats also lose bone mass (Miller &Wronski 1993). Consequently, absence of oestrogenmay be a contributory factor in lactation (Miller &Bowman 1998). This is supported by observations thatoestrogen treatment of ovariectomized lactating ratsimproved bone retention in lactation by 15% (Garneret al. 1991), but the mechanism here is confusedbecause these mothers also ate less and their pups grewless—so presumably other hormones related to lacta-tion (e.g. prolactin) were also at lower levels. Anotherhormone in addition to prolactin that appears


important in the supply of calcium to the foetuses isthe parathyroid hormone-related peptide (PTHrP;Yamamoto et al. 1991). PTHrP has strong sequencesimilarities to parathyroid hormone (PTH) and bothseem to interact with parathyroid hormone receptorslocated on osteoclasts, which they inhibit. PTHrPincreases during late pregnancy and remains highthroughout lactation (Sarli et al. 2005) where it isproduced in mammary epithelial cells. This promotesmaternal bone resorption allowing calcium release forexport in the milk. Production of mammary PTHrPappears to be sensitive to extracellular calcium levelsacting through the calcium-sensing receptor. Involve-ment of PTH itself in the calcium withdrawal processseems unlikely since parathyroidectomy had noimpact on the extent of bone resorption in lactation(Hodnett et al. 1992).

In addition to hormones that support milk pro-duction having impacts on bone, there is also evidenceof actions working in the reverse direction. For example,the osteoprotegerin ligand (OPGL) is a known osteo-clast differentiation and activation factor that is essentialfor successful bone remodelling. Transgenic micelacking OPGL (or its receptor) fail to develop lobularalveolar structures in their mammary glands resulting inan inability to secrete milk successfully. This effect canbe reversed by recombinant OPGL treatment (Fata et al.2000). Mammary and bone function therefore appearsto be intimately associated and this association, whichmay not necessarily be functional, leads to the obligatorybone loss during lactation. Following weaning, there is arapid increase in osteoblast proliferation and cancellousbone deposition (Miller et al. 2005). This osteopro-genitor pool may be primed during the late phase oflactation for this post-weaning activity. Peak boneformation in rats occurs approximately four weeksafter the end of lactation (Miller & Bowman 2004),restoring the bone lost during the lactation event(Nishiwaki et al. 1999; Bowman et al. 2002) and itsstrength (Vajda et al. 2001).

The physiological mechanism of lactation thereforeexposes the lactating female to an elevated risk ofosteoporosis, which may extend for at least twice theduration of the actual lactation event before levels arenormalized. The direct effect of this calcium withdrawalfrom bone is a reduction in bone strength and thisincreases the risks of bone fracture. Direct measure-ments of bone physical properties support this interpre-tation. By the end of pregnancy, rat tibiae were notsignificantly different in their strength from controls, butby the end of lactation, bone strength was substantiallyreduced in both cortical and cancellous bone, paralleledby the decline in bone volume (Vajda et al. 2001).

There are no direct measures of the increased risk offracture during lactation in small mammals. However,in humans, there are reports of case studies wherelactation-induced osteoporosis did lead to limb fracture(Clemetson et al.2004) suggesting that its effects may besignificant. Paradoxically repeated bouts of lactationappear to have a protective effect in humans on later liferisks of fracture attached to osteoporosis (Hoffman et al.1993; Jacobsen et al. 1998; Michaelsson et al. 2001;Petersen et al. 2002; Naves et al. 2005). This suggeststhat in the post-weaning period regrowth of bone is


overcompensated. There is some direct evidence tosupport this suggestion (Akesson et al. 2004).

(vi) Oxidative stressAnimals during lactation have high rates of metab-olism, which may result in elevated generation of freeradicals. These free radicals may cause oxidativedamage to macromolecules if the mice do notsimultaneously upregulate their oxidative defence andrepair mechanisms. Wiersma et al. (2004) showed thatbirds feeding nestlings, which is the most energeticallyexpensive phase in avian reproduction, do not upregu-late the levels of superoxide dismutase, catalase andglutathione peroxidase, the main defences againstradical oxygen species. However, that study did notinclude measures of oxidative damage—so the lack ofelevation of defence mechanisms may have beenbecause there was no increase in free radical productionto defend against. Indeed, the links between radicaloxygen species production and metabolic rate are notstraightforward (Speakman et al. 2002; Speakman2003), and in some cases increased metabolism mayactually reduce oxidative stress (Speakman et al. 2004).However, the reduction in levels of UCP-1 and UCP-3in lactation (above) would suggest that mitochondria inlactating animals are more closely coupled—support-ing the suggestion that this may be a time of elevatedoxidative stress (Demin et al. 1998a,b; Brand 2000;Speakman 2003). Studies of antioxidant protectionlevels and damage during reproduction are needed.

4. CONCLUSIONThe physiological costs of reproduction are probablythe most significant component underlying life-historytrade-offs. In this review, I have summarized some ofthe work that has been performed to measurephysiological costs in small mammals. Physiologicalcosts can be divided into two different aspects. The firstis direct costs of the reproductive event. These includethe energy and nutrient costs of reproduction and thechanges in organ morphology necessary to meet thesedemands. Considerable progress in elucidating thelimits on energy budgets in small domesticated rodentshave been made in the last 15 years or so. However,even in this area of intensive activity, we are still farshort of being able to translate this knowledge into anunderstanding of a simple life-history trade-off betweenthe number and size of offspring. Knowledge of thesedemands among non-domesticated species is verysparse, physiological information gleaned in a wildcontext is virtually non-existent. To these direct costsmust be added indirect costs of reproduction. Theseinclude optional compensatory adjustments thatanimals make to divert more resource into reproduc-tion. Such compensations include reductions inthermoregulation, physical activity and immunocom-petence. In some species, these adjustments can besufficient that total energy requirements during repro-duction are no greater than experienced by non-breeding animals. This may annul any ecologicalimpact of reproduction mediated, for example, viaincreased predation risk due to extra time spentforaging. Their long-term impacts have however


never been explored. To these compensatory costsmust be added obligatory costs, which are an inevitableside effect of reproduction that cannot be avoided. Forsmall mammals these include hyperthermia, bone loss,disruption of sleep patterns and oxidative stress.Knowledge of all these effects is presently rudimentary.Indeed, whether some changes are optional or obliga-tory (like immunocompetence changes) is uncertain,and the list of costs presented here is unlikely to becomplete.

Work performed by my own group described here wasperformed under UK Home Office regulations followingethical review and complied with local ethical guidelines foranimal experiments.

This paper was generated as part of the fitness function offood intake project funded by the Scottish ExecutiveEnvironment and Rural Affairs Department (SEERAD).

The work on MF1 mice described in this paper wassupported by BBSRC and particularly included work by ElaKrol and Maria Johnson. Many undergraduate and post-graduate students have also contributed to this work. I amgrateful to Ela Krol for her comments on the earlier drafts ofthis paper.

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http://dx.doi.org/doi:10.1095/biolreprod65.3.689

http://dx.doi.org/doi:10.1095/biolreprod65.3.689

http://dx.doi.org/doi:10.1079/PNS19830035


http://dx.doi.org/doi:10.1006/taap.1994.1168







http://dx.doi.org/doi:10.1002/(SICI)1097-010X(19990615)284:1%3C35::AID-JEZ6%3E3.0.CO;2-Z



http://dx.doi.org/doi:10.1037/h0077652

http://dx.doi.org/doi:10.1210/en.2003-0836



http://dx.doi.org/doi:10.1146/annurev.ecolsys.32.081501.114006

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The physiological costs of reproduction in small … physiological costs of reproduction in small mammals John R. Speakman* Aberdeen Centre for Energy Regulation and Obesity (ACERO),